Inorganic, organic and metallic substrates, such as glass, ceramics, concrete, plastics, iron and steel, aluminum and the like in three-dimensional shapes are used in many industries, such as packaging lines, construction of bridges, offshore drilling, oil and gas production, land, marine and air transportation, water and waste treatment, petrochemical production, agriculture, the manufacture of plastics, pulp and paper, the generation of power, and the like. The shapes can comprise sheets, drilling and processing piping, ship hulls, airplane parts, such as propellers and wings, highway and building girders, storage tanks, printing and paper machine rollers, and the like. Such shapes are routinely coated with polymers for a number of reasons, e.g., to impart water- and stain resistance; to decorate them; to improve ease of maintenance; to reduce friction; and the like. Less common, but no less important, reasons to polymer-coat substrates are to reduce sticking; e.g., of graffiti to glass, wood and ceramic tile; to reduce fouling, e.g., by marine organisms, such as by zebra mussels, algae and the like in marine tankers, inlet piping in power generation plants, and the like; and to provide easy removal of ice, e.g., from refrigeration equipment and the leading edges of airplane wings, stabilizers and propellers.
Illustrative of the state of the art of polymer-coating of such substrates is Law, et al., U.S. Pat. No. 4,113,665, who describe a binder curable at room temperature to produce chemically resistant coating for improving the resistance to corrosion, chemicals, solvents, weather and heat of steel structures used in chemical processing plants, oil refineries, coal fired power plants, offshore drilling platforms, and petroleum tankers. The binder comprises a silicone resin and a trialkoxy silane reacted with a polyol. Such coatings are useful to provide corrosion resistance, but they do not provide non-stick, foul-release and ice-phobic properties, for reasons not clearly understood at this time. It is believed that, in spite of the presence therein of silicone compounds, coatings made from such binders do not have a low enough surface energy to provide such characteristics, even for a short period of time, and especially not permanently.
Riddle, U.S. Pat. No. 5,039,745, describe a non stick coating composition for easing the removal of graffiti and sticky labels from buildings, the composition comprising a silicone resin, a poly(tetrafluoroethylene) resin and a polyurethane polymer in combination. The composition is said to resist removal by aggressive solvents, but it does not possess a low enough surface energy to permanently resist attack by marine organisms and to lower the resistance to ice removal substantially below values which are ordinarily achieved with the best state-of-the-art ice-phobic polymeric coatings, e.g., polypropylene, which itself is higher than desired at 60 lbs./in.sup.2.
Also of interest in this connection, is Mattor, et al., U.S. Pat. No. 4,282,054, who describe a composition for providing a release sheet for making laminates coated with a composition comprising a release agent, a cross-linkable thermoplastic resin and a water dispersible organic compound, some of which are surface active agents, containing polyoxyethylene, polyoxypropylene or a block copolymer thereof. The release agents used generally comprise a chrome complex of a fatty acid. Although the coating is described to be hard and smooth, and useful to preventing sticking of urethanes and polyvinyl chloride resins cast against sheets covered with it, the material is not easy to apply, requiring heating for curing, and it does not appear to be useful as an permanent anti-fouling coating and ice-phobic coating possibly because it does not possess a low enough surface energy and/or a contact angle (water) of 85 to 99.degree., preferably above 90.degree..
It has now been found that all of the shortcomings of the prior art are overcome according to the present invention if there is used a system comprising a primary coating having a surface energy in the range of 22 to 28 dynes/cm.sup.2 on the substrate and if there is interdigitated into the primary coating a top coating which has an even lower surface energy, of the order of 18 to 21 dynes/cm.sup.2. Interdigitation as used herein and in the appended claims defines using a means to interlock, e.g., like the fingers of folded hands, or couple, the two coatings together at their interface using, for example, a bi- or polyfunctional molecule one end of which interdigitates, or partially dissolves or chemically bonds to the primary layer and the other end of which binds to the non-stick polymer or prepolymer, either physically or chemically, and producing permanence not only to non-sticking but also to foul-releasing and ice-phobic characteristics. Such a novel system has been found to result in vast improvements in both the permanency and the efficiency of the foul-releasing and the ice-phobic properties, the former maintaining more than 10 months of effectiveness in sea water, and the latter reducing the removal resistance of ice on aluminum substrates to the order of 10 lbs/in.sup.2.